1. Field of the Invention
The present invention relates to a diffuser, and more particularly to a diffuser structure capable of preventing particles from being generated periodically in a plasma-enhanced chemical vapor deposition system, as well as a manufacturing method thereof.
2. Description of Prior Art
In a manufacturing process of a liquid crystal display device, a plurality of elements such as thin film transistors (TFTs) have to be manufactured on a glass substrate firstly. Please refer to FIG. 1, which illustrates the glass substrate 10 placed in a plasma-enhanced chemical vapor deposition (PECVD) reaction chamber 1. For composing the PECVD reaction chamber 1, a diffuser 12 is utilized as an upper electrode and a susceptor 14 is utilized as a lower electrode. The diffuser 12 comprises a plurality of throughholes (not shown) for gas or fluid to flow through. Plasma 16 is generated and filled between the diffuser 12 and the susceptor 14. When the chemical vapor deposition process is performed, required reaction gases are firstly conducted into the PECVD reaction chamber 1 through a gas inlet 18. Then, the reaction gases flow through a backing plate 20 and the throughholes (not shown) of the diffuser 12. Therefore, by manipulating a voltage difference applied between the diffuser 12 and the susceptor 14 and the effect of the plasma 16, desired films can be formed on the glass substrate 10 for manufacturing the elements. Finally, waste gases generated by the aforesaid manufacturing process are exhausted through a gas outlet 22.
However, the by-products left by the manufacturing process such as SiNx, amorphous silicon, polycrystalline silicon, SiOx, P—Si, and N—Si, are deposited on side walls of the throughholes (not shown) of the diffuser 12. When a film formed by these by-products is insufficiently stabilized, particles originated from these by-products will drop onto the glass substrate 10 and then adhere thereon. Once the aforesaid particles are too big or the particles are too much to drop onto the glass substrate 10, defects of the elements on the glass substrate 10 happen. This will lead to a bad performance of products of utilizing such glass substrate 10.
To solve the foregoing problem, a method of the prior art is to perform a periodic self-cleaning function for the PECVD reaction chamber 1 to remove the by-products deposited in the whole PECVD reaction chamber 1 for maintenance. The definition of such periodic self-cleaning function is that the deposited by-products are cleaned once after specific amounts of the glass substrates are processed. For example, the periodic self-cleaning function can be executed once for every 6 pieces of the glass substrates. In practice, a period of the periodic self-cleaning function can be executed between every 4 pieces and every 10 pieces.
Although the periodic self-cleaning function can solve the problem of the deposited by-products, a new problem may occur. In the beginning, the throughholes (not shown) of the diffuser 12 are formed by machining. Accordingly, the surface roughness of each side wall of the throughholes (not shown) is equivalent and the surface of each side wall is smooth. After the chemical vapor deposition process and the periodic self-cleaning function have been executed for some time, the surface roughness of each side wall of the throughholes (not shown) will increase. Then, when the periodic self-cleaning function is executed again, the particles will be formed on the surface of one of the glass substrates in the next processing period. For example, the periodic self-cleaning function is executed once for every 6 pieces of the glass substrates (Clean Count 1˜Clean Count 6). When the diffuser 12 is new, the periodic self-cleaning function may not have problems. However, after the diffuser 12 has been utilized for some time, the surface roughness of each side wall of the throughholes (not shown) will increase.
Hereafter, described is the new problem caused by the periodic self-cleaning function. Assuming that the periodic self-cleaning function had been executed for N times and before the (N+1)-th time periodic self-cleaning function, the phenomenon that enormous amount of particles adhered on the surface of some piece among the 6 glass substrates. For example, the particles drop and are adhered on the surface of Clean Count 2 glass substrate. The root cause of the aforesaid phenomenon is that after Clean Count 1 glass substrate was processed, the by-products are adhered on the side walls of the throughholes and form by-product films. Then, during the process of the Clean Count 2 glass substrate, the by-products drop off and become the enormous amount of particles because the film stress of the by-product films on side walls of the throughholes keeps accumulated and then exceeds the critical point. Consequently, the by-product films peel and form the particles, and the particles drop and adhere on the surface of Clean Count 2 glass substrate. After that, Clean Count 3 glass substrate is processed, the by-product films with lower film stress have peeled and the particles have already dropped and adhered on the Clean Count 2 glass substrate. Therefore the particles issue does not happen to the Clean Count 3˜Clean Count 6 glass substrates. In summary, as the surface roughness of each side wall of the throughholes (not shown) increases and the periodic self-cleaning function has been executed, the particles will be adhered on one specific glass substrate during the next self-cleaning period. To which glass substrate the particles issue happens depends on manufacturing processes and machine conditions of respective PECVD system.
Furthermore, it is noted that conditions of manufacturing processes of the PECVD reaction chambers provided by different manufacturers are different, thus the enormous amount of particles may not adhere to Clean Count 2 glass substrate specifically. The particles issue may happen to Clean Count 3 glass substrate or Clean Count 4 glass substrate, for example. However, the particles issue always happens to the same particular piece of Clean Count 1 glass substrate˜Clean Count 6 glass substrate. That is, the particles are periodically generated on the same particular piece of Clean Count 1 glass substrate˜Clean Count 6 glass substrate. This problem has been old and quite bothersome and annoying the entire industry field of LCD, but unable to be solved effectively for a long time. In such circumstance, as the size of the liquid crystal display panel is made larger and larger, the diffuser comes along with getting larger in size, and the cost of the diffuser is becoming more and more expensive as the same. The manufacturing cost must be too high if the diffuser has to be replaced once after executing every periodic self-cleaning function to solve the particles issue as a major option for the entire industry field of LCD.
Therefore, there is a need for developing a solution to the aforesaid problems that the particles are periodically generated after the surface roughness of side walls of the throughholes of the diffuser increases with executing the necessary periodic self-cleaning function.